1. Field of the Invention
The present invention relates to an anamorphic converter used with image-taking devices such as film cameras, television cameras, video cameras, or the like, for taking pictures with an aspect ratio which differs from that of the imaging device.
2. Description of the Related Art
Various techniques for recording and playback of images with conversion of aspect ratio have been conventionally proposed. Particularly, in the field of cinematography, a technique is widely used with a CinemaScope format (aspect ratio 2.35:1) picture recording/playback system wherein the picture is optically horizontally compressed using an anamorphic lens so as to be taken on film, and also shown at the time of playback by optically horizontally expanding the image on film using an anamorphic lens.
Known anamorphic converters include front converters which are attached to the object side of an image-formation optical system, such as disclosed in Japanese Patent Laid-Open No. 2-13916 and Japanese Patent Laid-Open No. 6-82691, for example. These converters are simple and do not exhibit vignetting because they ensure a suitable effective diameter regardless of the conversion ratio. Also, with regard to such front converters, Japanese Patent Laid-Open No. 3-25407 and Japanese Patent Laid-Open No. 5-188271, for example, propose techniques for correcting astigmatism due to focus.
Also, with regard to a rear converter to be attached to the image side of an image-formation optical system, the arrangement described in Japanese Patent Publication 3,021,985 (corresponding U.S. Pat. No. 5,307,084) with reduced change in astigmatism due to focus is known. Further, there is known a converter, such as described in U.S. Pat. No. 5,668,666, having a built-in converter and capable of being detachably inserted to the image side of the focusing group of the image-formation optical system. This built-in converter also is capable of reducing change in astigmatism.
Now, in recent years, video technology has seen a trend toward higher definition, to where digital cinema systems wherein cinematography carried out using HDTV systems are becoming commonplace. While imaging devices having an aspect ratio of 16:9 (1.78:1) are common with digital cinema systems, there is demand for an anamorphic converter for improving image quality by effectively utilizing pixels on the imaging device side in order to shoot pictures in the 2.35:1 aspect ratio CinemaScope format. Prerequisites for a cinematography anamorphic converter are that suitable aspect ratio conversion is performed, that there is no vignetting, that the effective image field of the image-formation optical system can be fully utilized, that there is little drop in light quantity at the periphery, and that high optical capabilities can be had over the entire zooming/focusing range of the image-formation optical system.
Now, although the front converter disclosed in Japanese Patent Laid-Open No. 2-13916 and Japanese Patent Laid-Open No. 6-82691 are advantageous in being simple, and not exhibiting vignetting due to ensuring a suitable effective diameter regardless of the conversion ratio, however, further improvements are desired with regard to larger size and change in astigmatism due to focusing. Also, the arrangements disclosed in Japanese Patent Laid-Open No. 3-25407 and Japanese Patent Laid-Open No. 5-188271 enable correcting of astigmatism due to focus. However, correcting means within the converter must be driven synchronously with the focusing of the image-formation optical system, thereby necessitating a complicated mechanism.
Also, the rear converter disclosed in Japanese Patent Publication 3,021,985 is advantageous in that there is no change in astigmatism due to focusing, but there is the need to suitably set the horizontal and vertical conversion scaling to suppress vignetting, and improvement is desired regarding change in field angle of the image-formation optical system. Further, the built-in converter disclosed in U.S. Pat. No. 5,668,666 also is advantageous in that there is little change in astigmatism due to focusing, but has a problem in that angular magnification is smaller than 1, and vignetting occurs.